In the later game, world number one Rafael Nadal defeated Novak Djokovic 7-5 6-2 after the Serb suffered eye trouble.

What had been an intense battle for more than hour in front of a sell-out O2 Arena suddenly became a stroll for Nadal after the world number three began to struggle with his vision.

The Spaniard now leads Group A with two victories, although nothing is yet decided in the group with all four players still able to qualify for the semi-finals.

Angry Andy

The former number one Andy Roddick railed against the umpire and destroyed a racket in frustration as a match he seemed to have under control slipped away.

Berdych, saved two set points at 4-5 in the first set and that proved to be a pivotal moment as his confidence came flooding back.

"I don't think he came into the match with a lot of confidence. Being able to get through that first set definitely raised his confidence level and he played well in the second set," Roddick told reporters of an opponent who had won just eight matches since reaching the Wimbledon final in July.

Having survived the set points, Berdych, who is the only debutant at the year-ending showpiece, changed gear and he broke the Roddick serve in the following game, crunching a forehand return past the motionless American off a 110mph second serve.

Berdych held serve to move a set ahead and Roddick was left smashing his racket against the sole of his shoe when he dropped serve again at 2-2 in the second, missing the sideline by a fraction with a smash after a superb defensive lob by Berdych.

Distracted

Roddick, who was upset by a digital advertisement changing colour in the middle of a point, never regained his composure.

Serving at 3-5 he pulled a limp forehand into the net on match point to hand Berdych a morale-boosting win.

"I'm happy after the first one to get back on the court and start to play my tennis again and feel good on the court," Berdych told reporters.

"That's what I need. I'm still in kind of chance to have it in my hands in the last match."

Berdych is the only player among the eight here not to have won an ATP title this season, but the last time he played in London he defeated Roger Federer en route to the Wimbledon final.

The 25-year-old's fairytale run came to an end in the final against Nadal and his form has dipped dramatically since then, so this victory offers a last chance to end the year on a high.

World number two Roger Federer leads Group B with two victories from Robin Soderling and Andy Murray, who both have one apiece. The top two places in that group will be decided on Thursday when Federer plays Soderling and Murray takes on Spain's David Ferrer.